The present invention concerns the preparation of glass fibers utilized for the reinforcement of plastic materials. In particular, it relates to a new sizing composition for coating glass fibers during their formation in the form of continuous filaments from molten glass or just after and also relates to the glass fibers coated with such composition.
More particularly, the invention deals with the preparation of glass fibers utilized for the reinforcement of sheets of molding composition. Such sheets are also known as Sheet Molding Compound (hereinafter referred to as SMC) from which products of reinforced plastic material are manufactured by means of heat and pressure.
The manufacture of articles of plastic material reinforced by glass fibers from SMC is increasing largely due to the low cost of this process in relation to other molding processes.
In the use of SMC, one or more flans cut out of a composite sheet of resin and cut glass fibers are placed on a mold in a press, and through heat and pressure the article is formed. Generally, there is utilized a flan whose surface corresponds to about 60% of that of the surface of the article to be molded, owing to the fact that the plastic material containing the glass fiber must flow in all directions in order to perfectly match the shape of the mold, in order to obtain a homogeneously molded product without mechanically weak points. Even more important than good mechanical properties, the article thus manufactured, generally sold as a raw molding, must have a faultless surface appearance which can be defined, on one hand, by the absence of visible fibers on the surface of the molded article, and on the other hand, by the absence of defects in the flatness of the surface, such as pinholes, craters, and flow waves.
The term "flow" will be utilized hereafter in order to designate the displacement of the plastic material during molding as well as the displacement of fibers or strands of cut glass, generally about 25 or 50 millimeters long.
The mechanical characteristics and the surface appearance of the reinforced plastics obtained in particular from SMC depend essentially upon specific qualities of the glass fiber utilized for reinforcement. However, the qualities are given to the glass fiber by its coating, therefore, generally through the sizing composition which is applied to it when it is in the form of continuous filaments, during its manufacture.
The treatment that the glass fibers utilized for reinforcement in SMC undergo during their manufacture when in the form of continuous filaments must therefore make them suitable for the reinforcement of plastics in general. That is to say, that it must not only join the filaments of the same strand, cover them with a thin lubricating layer in order to protect the filaments from mutual abrasion between them, and conserve the integrity of the strand during the winding and cutting operations, but also promote the binding between the glass and the synthetic resin in the composite.
The treatment must also give the strand the particular properties which make it suitable for the SMC application. In particular, the cut strand must keep its integrity during the maturation of the SMC. Thus, French Pat. No. 2,110,469 describes a sizing composition comprising in particular a modified epoxy resin, an emulsion of a copolymer of vinyl acetate and N-methylolacrylamide, and an emulsion of a copolymer of vinyl acetate and ethylene. This sizing composition gives the glass fiber certain properties already cited which make it suitable for the SMC application, in particular, a fairly good flow capability sufficient for the manufacture of small pieces. On the other hand, the fibers in accordance with this disclosure remain visible in the molded article. Furthermore, the flow capability is not sufficient for the manufacture of large pieces, owing to poor distribution of the reinforcement in the composite, hence, mechanically weak characteristics.
French patent application 77.23198 discloses a new sizing composition comprising in particular a copolymer with a base of vinyl acetate and acrylic monomers of which at least one has an epoxy function. Such composition produces a very good surface appearance for the manufactured article and a sufficiently regular distribution of the fiber in the article, especially when the article is of a very small size. Nevertheless, when one wants to manufacture articles of significant size, for example, on the order of a meter and longer, the flow of fibers, during the molding, is insufficient.
Sizing compositions recommended for the manufacture of large pieces from SMC are also known and are described, for example, in French Pat. No. 2,012,117 and U.S. Pat. No. 4,038,243. These sizing compositions have an unsaturated polyester resin base. They produce fairly good flow properties for the fiber. On the other hand, they give th strand a yellow coloration which subsists in the product after the molding process giving a poor surface appearance by making the strand extremely visible. This is why certain articles have their surfaces granulated or covered with a layer of paint.
Additionally, French Pat. Nos. 1,218,015 and 2,013,040 (U.S. Pat. No. 3,935,344) disclose sizing compositions for glass fibers utilized for the reinforcement of plastics in general and which have a base of alkyd resin capable of being cured by heating with or without the aid of melamineformaldehyde resin. With this composition, the sized strands do not possess attributes permitting them to flow readily and to reinforce the resin in a homogeneous manner in a large-sized article.
For uses other than the SMC application and even more generally for uses other than sizing of textile fibers, French Pat. No. 2,340,389 discloses a binding material for the reinforcement of webs or layers of glass fibers which themselves serve as binding material in the preparation of semi-finished products of thermoplastic synthetic resin with a polyacrylate base containing hydroxy and/or carboxyl groups and crosslinking agents such as blocked polyisocyanates, epoxy compounds, or products of aminoformaldehyde condensation. This binding material provides good resistance and good dimensional stability for the mats or layers of glass fibers.
The present invention provides an aqueous sizing composition comprising, in different proportions, components related to those of the binder composition of French Pat. No. 2,340,389. This sizing composition, utilized for the covering of continuous glass filaments during their manufacture from molten glass or just after, shows unexpectedly excellent qualities for textile sizing, in particular, excellent filmogeneous qualities. That is to say, that it spreads and distributes itself on the filaments in a very regular way; at the same time it gives great integrity to a strand. Even more, it gives a strand good rigitity or stiffness. In contrast to what one skilled in the art might assume from French patent publication No. 2,340,389, the strands are not inter-connected but isolated and perfectly adapted to the SMC application.
Furthermore, the present application involves the reinforcement by the said strands of thermosetting resins such as polyester resins, which are well-known by those skilled in the art as being very different from the thermoplastic resins described in the French patent publication No. 2,340,389.